Conventionally, a regulator of a motor vehicle alternator is put into service on the initiative of the driver by means of a contact key of the vehicle. With a so-called “mono-function” regulator, the regulator is connected to the contact key via a fault indicator lamp.
In more sophisticated regulators, such as so-called “multi-function” or controlled regulators, the regulator circuitry is supplied with power continually, but must go from a state of rest or standby, of reduced consumption, to an active state, when it is detected that the contact key is closed.
However, in order to obtain compatibility of wiring of these multi-function regulators with multi-function regulators, it is not possible to provide a direct connection between the contact key and the regulator, which makes it necessary to detect closure of the key by means of the fault indicator lamp.
In practice, the central problem of detection of this type consists of the distinction which must be made between the case in which the key is open and the lamp is off, in which case a terminal of the lamp on the regulator side is at the potential of a ground because of a pull-up resistance, and the case when the key is closed but, the lamp is lit because of the existence of a fault, in which case the terminal of the lamp on the regulator side is at a potential which is fixed, for example by a voltage drop of a semiconductor switching element which supplies the lamp with power. The smaller this voltage drop, the more the potential of the lamp terminal on the regulator side is close to that of the ground, which makes it difficult to distinguish between the open and closed states of the key.
In patent application FR2674063, the company VALEO EQUIPEMENT ELECTRIQUES MOTEUR solves this problem by implementing in a detection circuit a differential amplifier which controls the semiconductor switching element of the lamp by means of a difference between a voltage level of the terminal of the lamp on the regulator side, and a reference level, such as to maintain the potential of this terminal at a level which is sufficient not to deactivate the regulator when the alternator has a fault, but the contact key is closed.
The distinction between the open and closed states of the key is also made difficult by the existence of a leakage resistance between a terminal of the battery which supplies the lamp, and the terminal of the lamp on the regulator side which short-circuits the contact key.
A leakage resistance of this type, which is liable to appear in particular under the effect of conductive deposits, such as saline bridges which are created when the vehicle is exposed to salt spray, can have a value of 10 KΩ, or could be as low as 1 KΩ.
The variable nature of this leakage resistance according to environmental conditions makes a voltage divider bridge formed by this leakage resistance and the fixed pull-up resistance random, making detection of the open and closed states of the key based on a comparison at a single and fixed reference level unreliable.
In patent application FR2724268, VALEO EQUIPEMENT ELECTRIQUES MOTEUR solves this other problem by equipping a circuit for detection of the closure of a contact key for control of a battery charge regulator for an alternator with means for shunting parasitic leakage currents which circulate in this leakage resistance.
In addition, nowadays, ecological considerations are leading motor vehicle manufacturers to ask parts manufacturers to design alternators, an on-board battery charge voltage of which, fixed by a set voltage, is variable, such as for example to limit the torque collected from the thermal engine by the alternator, or to increase it according to a control strategy of an engine control unit.